Volunteers spotted scores of ghostly planes while hunting through satellite images for the missing Malaysia Airlines Flight 370. Many of these false positives stood out because they showed up as three separate brightly coloured airplanes, an interesting artefact of the way many satellites record a scene.

The imagery above, hosted by Mapbox, is from a satellite company called BlackBridge, which was among the many companies and governments that contributed data to the search. Their fleet of five satellites uses push-broom cameras to scan the Earth.

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These scanning systems are made up of long rows of sensors lined up perpendicular to the satellite's direction of motion, and act like a document scanner moving over a document. On some satellites, each row of sensors records a different spectral band. On the BlackBridge satellites, the rows record red, infrared, green and blue separately.

Push-broom satellite sensor arrays

Mapbox

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So, a fast moving plane is in a slightly different location as each row of sensors passes over it (at almost 5 miles per second), creating the illusion of three planes in the different colours. The red and infrared rows are paired on one CCD array, while the green and blue sensors are on a second array, which is why the red plane often seems much further ahead than the green and blue planes.

Searching through satellite data is laborious, which is why calls were made for the public to help spot possible clues. Humans are still far better than computers at identifying some kinds of objects in scenes, so as the search area expanded, more and more eyes were needed.

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The new southern Indian Ocean search area with the Chinese and Australian findings marked

Mapbox

Mapbox and BlackBridge have their latest imagery from the new search in the southern Indian Ocean (where the plane is now suspected to have gone down based on some interesting satellite calculations involving the Doppler effect), and you can help search for debris. The latest imagery (see at right) is really cloudy, but it will be updated as soon as new images are acquired.